1. Field of the Invention
The present invention relates to a method for the precise deposition of the energetic fills that form the detonation train within ultra-miniature safety-and-arming devices used for projected munitions, and more specifically wherein the crystalline high explosive materials which form the energetic fills are deposited using a combination of aqueous and non-aqueous vehicles.
2. Related Art
Modern explosive projectiles, such as mortar shells, artillery shells and other similar projectiles, normally have a safety-and-arming (S&A) device which operates to permit detonation of the explosive only after the particular projectile has been fired or launched. With the relatively recent emergence of smart weapon systems that are lighter, smaller, and have greater lethality and survivability, correspondingly smaller and more reliable S&A devices have been developed, including the development and use of Micro Electro-mechanical Mechanisms (MEMS) as S&A devices, such as the MEMS device disclosed in U.S. Pat. No. 6,167,809 to Robinson et al., issued Jan. 2, 2001, which is hereby incorporated herein by reference. Such MEMS S&A devices typically use a combination of mechanical mechanisms, which only under the extreme physical conditions of firing or launch create an alignment of very small explosive charges, a firing train, which when detonated by the weapon's fuze system will allow the detonation of the projectile's main charge. As disclosed in U.S. Pat. No. 6,167,809, MEMS S&A devices are preferably fabricated on a die approximately one square centimeter or less in area, wherein the very small explosive charges, micro-liter volumes, must to be precisely deposited within a series of holes and channels that comprise the firing train.
Typically, within MEMS devices, there are primary and secondary explosives; where primary explosives are very sensitive explosives that respond to a small “insult,” while secondary explosives usually require a strong shock to detonate. The primary explosives within a MEMS fire train are used to initiate the train, being detonated by a small electrical charge from the fuze circuitry of the projectile. Methodology for depositing the requisite very small quantities of primary explosives along a bridgewire are known. However, the balance of the fire train within the MEMS device is filled with a secondary explosive to minimize the potential for a premature detonation. This secondary explosive, filled within the holes and channels which form the firing train of the MEMS device, is the subject of this invention.
The basic standard methods for loading secondary energetic or explosive materials into munitions are press-loading, and cast loading (whether using melt-cast or cast-cure techniques). The obvious alternative method of loading explosives as a slurry, is not economically feasible due to the excessively long drying time to evaporate the slurry medium—which if not fully evaporated can lead to defects such as porosity, voids, cracks and entrapped slurry medium and the like, which can cause a fielded munition to have safety and performance problems.
Press-loading, as the means to deliver the explosive to the fixture, presents difficulties because of the very small volume of solid explosive required in each MEMS hole or channel. To obtain the desired explosive effect along the initiation train, compression of this explosive material is critical and generally a density per unit volume of greater than 95%, i.e. 95% of the Theoretical Maximum Density (TMD), is required. Such a high minimum density avoids cracks, porosity, voids, and the like, which can result, as stated above, in safety and performance problems. Further, because of the delicateness of the materials of construction of the ultra-miniature MEMS fixture, press loading of the energetic fill into the fixture to meet this high minimum % TMD is not a viable option.
One alternative potential approach would be to prepare a pellet of the energetic material externally of the fixture, and then load the pellet into the fixture. To complete the process, in order to maintain the pellet in place, some kind of adhesive would have to be applied to the pellet, e.g., on the side thereof, or to the wall of the fixture. It will be appreciated that such a process would be difficult due to being cumbersome and relatively costly.
As was also mentioned previously, another alternative is casting of the energetic fill into the fixture, either by melt casting or cast curing. Melt casting basically entails heating a substance to a temperature above its melting point, adding any needed ancillary materials to the melt, pouring the mixture into the volume to be filled, and allowing the fill to solidify in place. Among other problems with this approach, because of the very small delivery volumes involved in the firing train within MEMS devices, heat loss to the ambient environment would be a problem and, in this regard, can result in the energetic material beginning to solidify before being emplaced.
Cast curing basically entails mixing the substance to be cast in a liquid polymer mixed with a cross-linking reagent. The resultant cast mixture has a finite “pot life” after which the viscosity of the mixture increases due to the process of chemical crosslinking. This change in rheological properties can cause difficulty in the delivery into the fixture of energetic material prepared in this manner.
There are, of course, a number of state-of-the-art delivery devices for the delivery of small volumes of materials including ink jet printing. The latter is a mature technology that can be used to accurately deliver small volumes of material. However, the present technology is unsuitable for delivering energetic materials for two reasons. First, most inks used for ink jet printing are dye-based, i.e., the colorant dye is dissolved in the fluid medium, and although there are pigment-based ink jet inks available wherein the colorant is an undissolved crystalline material, the undissolved solids are of a sub-micron particle size. Important secondary high explosives such as CL-20 (epsilon HNIW) are not presently available in a sub-micron particle size. Further, in an ink jet printer, the ink is typically delivered from the print head by a piezoelectric discharge that ejects droplets of ink at elevated pressure and temperature onto the printing substrate; the combination of an electric discharge and high temperature/pressure can be a safety hazard when attempting to deliver energetic materials.
Considering the above factors, there is a need in the art for a method to effectively, safely, and precisely load explosive charges, in the micro-liter volumes, into the holes and channels that comprise the firing train within MEMS S&A devices.